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BUREAU OF MINES 
INFORMATION CIRCULAR/1990 

¥9f 




The Bureau of Mines Ground-Fault 
Protection Research Program-A 
Summary 

By M. R. Yenchek and A. J. Hudson 




YEARS a 

***u of ^ 



U.S. BUREAU OF MINES 
1910-1990 

THE MINERALS SOURCE 



Mission: As the Nation's principal conservation 
agency, the Department of the Interior has respon- 
sibility for most of our nationally-owned public 
lands and natural and cultural resources. This 
includes fostering wise use of our land and water 
resources, protecting our fish and wildlife, pre- 
serving the environmental and cultural values of 
our national parks and historical places, and pro- 
viding for the enjoyment of life through outdoor 
recreation. The Department assesses our energy 
and mineral resources and works to assure that 
their development is in the best interests of all 
our people. The Department also promotes the 
goals of the Take Pride in America campaign by 
encouraging stewardship and citizen responsibil- 
ity forthe public landsand promoting citizen par- 
ticipation in their care. The Department also has 
a major responsibility for American Indian reser- 
vation communities and for people who live in 
Island Territories under U.S. Administration. 



/information Circular 9260 

The Bureau of Mines Ground-Fault 
Protection Research Program-A 
Summary 



By M. R. Yenchek and A. J. Hudson 

ii 



UNITED STATES DEPARTMENT OF THE INTERIOR 
Manuel Lujan, Jr., Secretary 

BUREAU OF MINES 
T S Ary, Director 



ILLUSTRATIONS - Continued 



Page 



15. Sensitive CT design 8 

16. Electronic relay and CT 9 

17. Prototype sensitive GFR enclosure 9 

18. Block diagram for prototype ac analog relay 9 

19. Schematic diagram for prototype ac analog relay 10 

20. Block diagram for prototype digital relay 10 

21. Schematic diagram for prototype digital relay 11 

22. Bruceton Mine power system 12 

23. Power feed through in A-Butt, Bruceton Mine 12 

24. Load center in 12-Room, Bruceton Mine 12 

25. Sensitive GFR installation, Bruceton Mine 13 

26. Totalizer amplifier circuits for digital and analog GFR's 14 

27. Differential current relaying using saturable transformer 15 

28. Saturable transformer current sensor 16 

29. Saturable transformer output voltage versus dc fault current 16 

30. Drop in sensor output versus frequency 16 

31. Saturable transformer prototype 17 

32. Dc relay block diagram 17 

33. Dc relay timing diagram 17 

34. Schematic diagram of dc relay prototype 18 

35. Dc relay prototype 19 

36. Typical high-voltage distribution circuit 19 

37. Simplified diagram of ground-check monitoring system used in mine distribution systems 20 

38. System block diagram 21 

39. Block diagram of relay circuit 21 

TABLE 

1. Counter readings of GFR performance 13 





UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT 


A 


ampere 


kQ 


kilohm 


mV 


millivolt 


dB 


decibel 


kV 


kilovolt 


Q 


ohm 


op 


degree Fahrenheit 


lb 


pound 


pet 


percent 


ft 


foot 


mA 


milliampere 


s 


second 


h 


hour 


pF 


microfarad 


V 


volt 


hp 


horsepower 


fiU 


microhenry 


VA 


volt-ampere 


Hz 


hertz 


MQ 


megohm 


W 


watt 


in 


inch 


ms 


millisecond 


Wb/A-m 


weber per ampere-meter 


kHz 


kilohertz 


MS 


microsecond 


yd 3 


cubic yard 



THE BUREAU OF MINES GROUND-FAULT PROTECTION 
RESEARCH PROGRAM-A SUMMARY 



By M. R. Yenchek 1 and A. J. Hudson 2 



ABSTRACT 

The U.S. Bureau of Mines designed and constructed sensitive and coordination-free ground-fault relays 
(GFR's) for use on mine power systems. First, a list of GFR attributes for mine ac utilization applications 
was compiled. These practical guidelines specified design, construction, transient immunity, reliability, and 
operating criteria. The time-current characteristics of the ac and dc units, subsequently fabricated, were 
designed to be below the human electrocution threshold. The significant and highly variable capacitance of 
high-voltage distribution cables was found to preclude the sensing of ground currents in the milliampere 
range. However, the coordination-free system developed for high-voltage distribution should enhance safety 
by significantly decreasing response time to ground faults. Implementation of the sensitive GFR technology 
in the mining industry has the potential to eliminate the majority of injuries and nearly all the deaths resulting 
from contact with energized components. 



Electrical engineer. 
2 Electronics technician. 
Pittsburgh Research Center, U.S. Bureau of Mines, Pittsburgh, PA. 



INTRODUCTION 



An analysis of U.S. Mine Safety and Health Ad- 
ministration (MSHA) statistics for electrical accidents 
at underground mines between 1980 and 1985 showed that 
electricians and mechanics suffered the highest incidence 
of injury and death. Specifically, there were 595 acci- 
dents associated with repair and maintenance of electri- 
cal apparatus, 29 of which were fatal. The resultant 
cost to the public and private sectors was estimated at 
$28 million (i). 3 

These data are not surprising since electricians and 
mechanics have the greatest exposure to electrical hazards. 
Space on mining machines is extremely limited, with 
electrical control boxes crowded with parts. During 
troubleshooting of energized circuits there is danger that, 
through either inattention or an inadvertent slip, an elbow 
or arm will contact an energized component. Under such 
working conditions electrical safety can be improved 
through the use of effective protective devices for 
personnel. 

When a worker contacts an energized component, 
electric current flows through the worker's body and 
returns to the power source, either through the earth or 



via a ground wire. In this case the presence of a ground 
wire does not preclude or mitigate hazardous leakage 
current. The only available safeguard in such occurrences 
is a ground-fault detection system. 

Most ground-fault protection in use on underground 
mine power systems is inadequate from a shock prevention 
standpoint. Typical response levels are in the ampere 
range, significantly exceeding the electrocution threshold. 
Increasing device sensitivity results in undesirable nuisance 
tripping and unscheduled interruptions in production. 
However, a sensitive ground-fault relay (GFR) not only 
can identify and act to interrupt the small deadly ground 
currents that electrocute people, but can ignore spurious 
signals that may result when motors are started or circuit 
breakers switched. Recent U.S. Bureau of Mines research, 
aimed at virtually eliminating electrocutions resulting from 
direct contact, developed sensitive GFR's for ac and 
dc utilization circuits. In addition, coordination-free relays 
were devised for use on high-voltage distribution. Ap- 
plication of this technology in the mining industry could 
eliminate the majority of injuries and nearly all deaths 
resulting from contact with energized components. 



ELECTRIC-SHOCK ANALYSIS 



Although the prevention of sustained electric shock is 
an ideal goal for industry, it is usually impractical. The 
detection of such shocks and resultant body currents as 
low as 10 mA would likely be a complete impediment to 
production. Consequently, a more realistic goal is to 
design protection against electrocution, not electric shock. 

To devise effective personnel protection, it is first 
necessary to understand how electrical current can be 
lethal to humans. Ventricular fibrillation is by far the 
most common cause of death from accidental electric 
shock. This condition is induced when sufficient current 
flows through the chest and disrupts the nervous system 
impulses, internal to the heart, that synchronize normal 
heartbeat. The heart no longer acts as an efficient pump 
to circulate blood, and death is likely to occur within 
minutes. In light of this, safety can be enhanced if the 
potential current flow through the body can be minimized. 

This risk of electrocution is determined to a large ex- 
tent by power system configuration. To maximize safety, 
the recommended arrangement for ac systems features a 
direct- or derived-neutral point on the source trans- 
former secondary, tied to earth through a grounding 
resistor. Equipment frames are then grounded by a 



grounding conductor connection to the grounded side of 
the resistor. When a grounded worker accidentally 
contacts an energized conductor, the body current is 
limited in magnitude by the grounding resistor (fig. 1). 

Ventricular fibrillation is a function not only of current 
magnitude (I), but also of frequency, duration of expo- 
sure (t), and weight of the victim. The threshold was 





Italic numbers in parentheses refer to items in the list of references 
preceding the appendix at the end of this report. 



Figure 1 .-Resistance-grounded power system with grounded 
worker. 



statistically defined by Dalziel and Lee (2) as that current 
through the chest that will cause ventricular fibrillation in 
1 out of 200 people. For 110-lb individuals, the 60-Hz 
threshold was expressed as 



I = 116/ Ji, 



(1) 



where I = current, mA, 



and 



t = time, s. 



This relationship is shown graphically in figure 2. For 
brief exposures, relatively more current is needed to cause 
fibrillation. For longer durations, the limit decreases, 
down to 50 mA, below which fibrillation is unlikely no 
matter how long the exposure. 

Given these constraints, an optimum ohmic value can 
be determined for the neutral grounding resistor, R g , 
shown in figure 3. This value should be high enough to 
protect against ventricular fibrillation, yet sufficiently low 
to permit reliable ground-fault detection without nuisance 
tripping. Ideally, the resistor should limit current below 
the electrocution threshold for a direct contact shock. In 
such instances, the body resistance, R b , may be as low 



350 




4 5 

Figure 2.-60-Hz fibrillation threshold for 110-lb individuals. 



as 500 (3). The equation defining the 60-Hz electro- 
cution threshold may be rewritten to determine the 
maximum nonfibrillating current for the total circuit 
clearing time as follows: 



where 
and 



I = 116/7(1! + t 2 ) , 
tj = operating time of GFR, s, 



(2) 



t 2 = operating time of molded-case circuit 
breaker, s. 



A relay operating time of 100 ms provides sufficient time 
delay to prevent nuisance tripping. A molded-case circuit 
breaker typically opens its contacts within 34 ms. Given a 
total operating time of 134 ms, the maximum current that 
will not result in fibrillation is 317 mA. This can now be 
used to define the ohmic value of the neutral grounding 
resistor, R (fig. 3): 



R g = (V in /I)-R b , 
where V- = line-to-neutral system voltage, V, 



(3) 



and 



I = maximum nonfibrillating current, A, 
R b = human body resistance, ft. 



For example, on a 480-V system, R g = 374 ft, and ground 
faults are limited to a maximum of (480/75) /374 = 
740 mA. 

The human response to currents of varying frequency is 
shown in figure 4 (4). It is an unfortunate fact that 
humans are most sensitive to 60-Hz signals. The reason 
for this is that, physiologically, the muscles and the nerves 
of the body are most easily stimulated by changes in 
current magnitude. The ac sine wave is characterized by 
constantly changing magnitude, as opposed to pure dc 
where the only change occurs the instant the circuit is 
made or broken. Consequently, about 3.5 times more dc 
than ac is required to induce ventricular fibrillation (5). 
Conversely, the muscles and nerves do have a finite 
reaction time, such that with increasing frequency, the 



V|n© 




Rb 



Figure 3.-Electrical accident equivalent circuit 



stimulation of one alternation does not have time to elicit 
a response before it is annulled by the succeeding 
alternation. 

For dc power systems, a three-phase bridge rectifier 
arrangement is preferred (fig. 5). Shock hazards are 
reduced, not only by the presence of the grounding re- 
sistor, but also because line-to-ground voltage is one-half 
the line-to-line dc voltage. When a grounded worker 
inadvertently contacts an energized positive or negative 
conductor, the resultant current through the individual is 
half-wave rectified as shown in figure 6. Research con- 
ducted recently by Bernstein (<5) has established that the 



presence of an 18-pct ripple in a half-wave-rectified wave 
tends to reduce the threshold such that the following 
relationship applies (fig. 7): 



i = 348/yr, 

where I = current, mA, 



(4) 



and 



t = time, s. 



These electrocution threshold characteristics are an im- 
portant consideration for the design of sensitive GFR's. 



600 




10 100 1,000 10,000 

FREQUENCY, Hz 

Figure 4.-Fibrillation threshold at currents of varying 
frequency. 



m 




ooo_ 




til 



Ripple frequency = 180 Hz 




TIME 



Figure 6.-Half-wave-rectified ground-fault current 



900 




Figure 5.-Three-phase resistance-grounded system feeding 
full-wave rectifier. 



"012345 
Figure 7.-Half-wave-rectified dc ventricular fibrillation threshold. 



ALTERNATING CURRENT UTILIZATION 



Initial Bureau research concentrated on providing the 
ac utilization portion of a mine power system with sensitive 
ground-fault protection. The utilization system includes 
portable power cables, power centers, rectifiers, motors, 
and the associated protective devices. It is the most 
troublesome part of the power system in terms of safety 
and reliability because of its temporary nature. As mining 
progresses, the utilization system is stretched to its limit 
and repositioned, necessitating frequent handling of trailing 
cables and equipment repairs, a major source of electro- 
cutions underground (1). The presence of large motor 
loads strains the dependability of protection devices by 
introducing large voltage and current transients. Con- 
sequently, the application of effective personnel protection 
for utilization circuits would have a major impact on 
underground safety. 

Zero-sequence or balanced-flux relaying (fig. 8) is 
the most reliable and most common method employed for 
ground-fault relaying (7). As shown in figure 8, the phase 
conductors pass through the toroidal current transformer 
(CT) window. The sum of the three phase currents is the 
CT primary current and is proportional to the zero- 
sequence current (8). An unfaulted balanced system 
features little or no zero-sequence current, and the CT 
secondary current is approximately zero. However, when 
a ground fault occurs, the resultant secondary current is 
used to trip the relay. Zero-sequence relaying is 
unaffected by phase voltage fluctuations, and since only 
ground leakage is monitored, the relay can be made very 
sensitive. Consequently, it is the only practical technique 
capable of responding to low-level hazardous ground 
currents and the method of choice for the Bureau's 
sensitive GFR research program. 

BACKGROUND RESEARCH 

As a first step in the design process, a list of GFR 
attributes for ac mining applications was compiled. Next, 
test procedures were devised to ensure device compliance 



Phase 
A — 



B- 
C- 




ZN) sequence 
relay 



Ground wire 



Figure 8.-Zero-sequence relaying. 



with the desired operating characteristics. These practical 
guidelines (9), addressing GFR design and construction, 
transient immunity, reliability, and time-current char- 
acteristics, are summarized below. 

Proper Design and Construction 

A relay system of suitable design and construction does 
not pose a personnel hazard, reduces the amount of down- 
time caused by GFR failures, and facilitates acceptance of 
the GFR as a useful safety device. Electronic instruments 
designed and constructed for military use must comply 
with Military Standard 454 (10). The key portions of that 
standard, which can be applied to GFR's in underground 
mining, are related to safety and accessibility: The design 
shall incorporate methods to protect personnel from 
accidental contact with voltages greater than 30 V root- 
mean-square (RMS) or dc during normal operation. All 
external surfaces shall be at ground potential during nor- 
mal operation. All terminals shall be corrosion resistant. 
Sharp external projections shall be avoided. Suitable ac- 
cess shall be provided for adjustments, testing, and routine 
maintenance. No unsoldering shall be necessary to remove 
the front cover for troubleshooting. 

Mine Worthiness 

Underground, GFR's are located inside metal-clad load 
centers, so both the relay and CT must have metal mount- 
ing lugs. Terminal strips should be sized for No. 12 AWG 
wire. In addition, the relay case should be moisture and 
dust resistant. 

Size Limitations 

Space is typically limited inside mine power equipment 
compartments. Since several GFR's may be used in a sin- 
gle power center to protect all outgoing circuits, they must 
not be much larger than present GFR's. Thus, the relay 
components should be mounted in a compact enclosure 
not exceeding 3 by 6 by 6 in. To minimize flux leakage, 
the CT window should only be large enough to accommo- 
date the encircled power conductors. The outside dia- 
meter of a 4/0 single-conductor cable is 0.807 in (11). 
Three such cables fit snugly through a 1.750-in-diameter 
window. For ease of installation of cables with terminals, 
the window diameter should be increased to 2.100 in. 
Present ground-fault CT's in use underground have outside 
diameters smaller than 4 in. Since they are placed 
between the molded-case circuit breaker and the load- 
center coupler, they are no more than 3 in wide. 

Electrocution Prevention 

The primary reason for employing sensitive GFR's in 
mining is to prevent accidental electrocutions. For 60-Hz 
circuits, the desired region for GFR operation is below 



and to the left of the threshold shown in figure 2. To 
define GFR time-current characteristics, a variable 60-Hz 
voltage source in series with a 50-fl, 225-W fixed resistance 
is used to inject current through the CT primary as shown 
in figure 9. A double-pole, single-throw switch initiates the 
test and triggers the storage oscilloscope, which monitors 
relay contact activity. Test currents are varied from to 
1,000 mA. 

Power Harmonics 



GFR CT, should not falsely activate the relay. In addition, 
they should not damage the relay control circuitry. 

An impulse generator, constructed in accordance with 
Underwriters Laboratories (UL) Standard 943 (13), is em- 
ployed to simulate transient overvoltages as they would 
occur on residential and industrial power systems. The 
test circuit consists of a relay switch and resonant circuit, 
shown schematically in figures 11 and 12. The generated 
waveform exhibits the following characteristics under no 
load: 



The filtering for GFR's must be designed so as to pre- 
clude false tripping by any harmonics superimposed on 
the power conductors. However, attenuation of these 
higher frequencies must not be so severe that hazardous 
currents above the electrocution threshold (fig. 4) are 
permitted. In testing GFR frequency response (fig. 10), an 
audio oscillator and power amplifier provide high- 
frequency currents from 60 Hz to 10 kHz. For each 
frequency, the voltage is slowly increased until the GFR 
activates. 

Voltage Surge Immunity 

Mine power systems frequently experience voltage 
surges when circuit breakers and switches are opened or 
closed. Although the duration of these transients is less 
than 50 ms, past research indicates their magnitude can 
reach five times the utilization voltage (12). These surges, 
when present on the power conductors encircled by the 



Resistor 
— WV 



L 



— -To oscilloscope 
v. trigger 

A 



(£) Voltage 
V- / source 



..Current probe 
to oscilloscope 
channel 2 



JT 

T_ 



To channel 1 



Relay 
Figure 9.-Test setup to determine 60-Hz tripping characteristics. 



1. Initial rise time of 0.5 ps between 10 and 90 pet of 
peak amplitude; 

2. Period of following oscillatory wave, 10 /is; and 

3. Amplitude of each successive peak, 60 pet of the 
preceding peak. 

The amplitude of the first peak is fully adjustable from 
to 9,000 V. In the first part of the test, 10 successive 5- 
kV surges are imposed on the power conductors encircled 
by the CT while the relay contacts are observed. Next, ten 
1-kV impulses are applied in parallel with the 120-V ac 
control voltage, and at random with respect to its phase. 
Afterward, the relay is operated at 60 Hz to detect 
possible damage to circuitry. 



Line output 




Relay 
control 



rrn 

Cathode-ray oscilloscope 
trigger output 



Neutral ground 



Figure 11. -Impulse generator circuit 



CT 



Frequency 
counter 



Frequency 
generator 



\®^n =z 7^. 



<£> 



Ammeter 
Figure 1 0.-Frequency response test circuit 



Light 



I20V 
60 Hz 



II, 50 V A 



lOkil, IW D l 



-w* ►Hr — t 



32 M F, 250 V 



11* >' 



CR| relay 

Cathode-ray 
oscilloscope 
gate input 
R 3 
I kfl, '/feW 




I M.a, 7 2 W 



rm 



Figure 12.-Relay control circuit for impulse generator. 



Common-Mode Transients 



Safe Failure Modes 



Sensitive GFR's must be unaffected by large transient 
currents occurring simultaneously on all phases of an ac 
utilization circuit. Such currents may briefly exceed six 
times full-load motor rating during starting or under heavy 
intermittent load. The maximum short-circuit settings 
listed in 30 CFR 75.601 (14) effectively limit balanced 
three-phase loading to 2,500 A. Nevertheless, balanced 
currents up to 2,500 A should be tolerated for up to 5 s 
without activation of the GFR. A three-phase high-power 
source is used to variably supply balanced three-phase 
currents through a shorted trailing cable encircled by the 
GFR CT. The voltage is increased until the relay activates 
or the 2,500-A ceiling is reached. Tripping thresholds are 
confirmed through repeated tests. 

Current Withstand 

The molded-case circuit breakers used on low-voltage 
ac mine power circuits typically have an interrupting rating 
of 30,000 A. Currents of this magnitude are quite possible 
during three-phase faults. Since the GFR CT is a part of 
the power system, it too should withstand up to 30,000 A 
for the time it takes the breaker to clear (a few hertz). 
The withstand test is conducted with a high-current circuit 
breaker tester as shown in figure 13. The tester is 
equipped with an initiate switch that can be jogged to 
reasonably control the test duration. Current magnitudes 
are recorded on a storage oscilloscope connected across a 
400-A, 100-mV shunt. The CT secondary is shorted to 
preclude high secondary voltages. The CT is subjected to 
30,000 A for approximately 4 Hz. The 60-Hz current ratio 
and winding resistance are measured before and after the 
withstand test to detect any degradation of the CT. 

Quality Assurance 

For dependability underground, all devices from the 
same manufacturer should be consistent in electrical and 
mechanical performance over a reasonable service life. In 
addition, each GFR should be equipped with a means to 
test its operation. 



In the event of failure of the GFR's internal circuitry, 
it is vital that the unit activate its associated circuit breaker 
to remove power and prevent a false sense of security. 
Two common failure modes are the loss of 120-V control 
power to the GFR and an opening of the CT winding. 

PROTOTYPE DESIGN AND CONSTRUCTION 

Sensitive GFR's are available commercially for use in 
British mines and in the U.S. irrigation industry. Three 
such brands were evaluated in-house for potential appli- 
cability to U.S. mining using the above criteria. None was 
found suitable without design modifications (15). How- 
ever, these tests served as the basis for the development of 
a mine-worthy sensitive ground-fault protection system 
consisting of a CT and an electronic relay. 

Current Transformer 

The CT for a sensitive ground-fault system has the duty 
of precisely sensing the existence of small ground-fault 
currents on three-phase line conductors feeding mine ma- 
chinery. It must be able to distinguish these faults within 
the complex electromagnetic environment that exists in 
mine power equipment. Specifically, the CT must not send 
erroneous signals to the relay circuitry when in the pres- 
ence of external magnetic fields and when observing high 
common-mode line currents. This precise device must also 
be able to physically, electrically, and magnetically with- 
stand the mine environment. 

The output of the CT secondary was anticipated to 
be in the millivolt range when sensed currents were in 
milliamperes. The following expression is used to pre- 
dict open-circuit output voltage for a given ground-fault 
current (16): 



E = 



= hNuflplnCR^Rj) 



(5) 



where E = RMS output voltage of secondary volt- 
age, V, 

h = toroid core width, m, 



To storage 
oscilloscope 



Mine duty 
circuit breaker 




Secondary 
shorted 



Figure 1 3.-Current withstand test circuit 



and 



N = number of turns on secondary winding, 

u = core permeability, Wb/A-m-turns, 

f = frequency, Hz, 

I p = RMS value of ground-fault current, A, 

R Q = outer toroidal radius, m, 

Rj = inner toroidal radius, m. 



Equation 5 is valuable in designing a current sensor for a 
sensitive ground-fault protection system. It shows that CT 



output voltage may be affected by various electrical and 
mechanical quantities. 

In addition, consideration must also be given the CT 
burden, or external load impedance on its secondary. The 
burden impedance cannot be so low as to reduce the sec- 
ondary voltage to an unusable value. Voltage input to the 
relay must be sufficiently high so that amplification and 
noise concerns are minimal. Since core dimensions are 
limited by available space, voltage output can be maxi- 
mized by using a high-permeability core and a large 
number of secondary turns (see equation 5). Also, a high- 
permeability core minimizes flux leakage inherent with 
window-type CT's. 

Cores are usually constructed of thin laminations to 
reduce eddy current losses. For the GFR toroid, the core 
was made by continuously winding a 0.006- by 0.75-in tape 
of magnetic material starting with a 2.5-in ID. After 
winding to an outer diameter of 3.5 in, the core was 
annealed to remove strains and impurities from the tape. 

An 80 pet nickel-iron alloy core was able to meet the 
accuracy requirements of sensitive ground-fault relaying. 
However, one problem with nickel-iron cores is that then- 
excellent magnetic properties can be degraded by winding 
stresses and pressures. A nonmetallic or phenolic box 
(fig. 14) around the core provided protection against 
mechanical damage as well as insulation for the secondary 
winding. The effects of vibration and shock are mitigated 
by cushioning the core in a silicon casing. 

The CT must be able to operate within significant mag- 
netic fields generated by nearby power transformers or 
the monitored power conductors themselves. To minimize 
any volts-per-turn imbalance requires a regressive winding 
technique (fig. 15) (17). One-half of the 250 secondary 
turns of No. 22 AWG magnet wire were wound around the 
core in one direction. The second half were wound 
around the core in the opposite direction across the first 
half so that each second-half turn falls between a pair of 
first-half turns within the window crossing on the CT sides. 
A separate single turn was wound on the core for primary 
injection testing. 

As stated previously, power conductor common-mode 
currents can produce local core saturation, noncancelling 
voltages across the CT secondary, and nuisance tripping of 
the relay. It was found (18) that a 2.1-in-ID, 0.1-in-thick, 
concentric, low-permeability iron buffer adjacent to the 
power conductors tended to distribute local flux saturation 
effects. 

The core, windings, and buffer were potted in epoxy 
and enclosed in a metallic housing that has convenient 
holes for mounting the assembly inside mine power equip- 
ment. The output cable contains the four conductors for 
the CT secondary and the test winding. 

Electronic Relay 

Solid-state sensitive GFR's were designed to operate 
with the CT (figs. 16-17). Two versions were conceptu- 
alized, one based upon analog techniques, the other using 



Nonmetallic 
insert 




Nonmetallic 
case 

Insert 
cushioning 



Tape- 
wound 
core 



Figure 14.- Nonmetallic core box construction. 



Toroid core, 2.5-in-ID, 3.5-in-OD, 0.75-in-high, 

tape-wound with 0.006-in nickel- iron 
alloy, cased and phenolic boxed. 




250-turn, regressively 
wound CT secondary 



Transformer 
secondary leads 



Test winding leads 



-2.l-in-ID low-permeability steel pipe 
with single wrap of 0.00l-in nickel- 
iron alloy on outer surface. 

Figure 1 5.-Sensitive CT design. 



digital (19). Both incorporate the same control power 
supply and electromechanical relay trip circuitry mounted 
on interchangeable cards. 

Analog Version 

A block diagram of the analog relay is shown in fig- 
ure 18 and a schematic is shown in figure 19. Since the 
sum of the three line currents that form the primary cur- 
rent in the CT is nearly zero under normal operating con- 
ditions, normal phase currents should not trip the relay. 
However, when a ground fault occurs, a voltage will be 
induced across the secondary in proportion to the fault 
magnitude. The CT (T 2 ) secondary was connected across 
a burden resistor (Rj) and an inverting operational ampli- 
fier QA : (fig. 19). Diodes D 1 and D 2 were included to 
limit short-duration transients at the GFR input. 

A first-order, low-pass filter was utilized to match the 
relay frequency response to that of humans (fig. 4). This 
filter consisted of operational amplifier QA 2 , resistors, R 5 




♦ 




12 3 

i 1 L_J 



Figure 16.- Electronic relay and CT. 



h 2 - 32 "H 

0.596" typical 
0.86" typical |— \ 

-E5B 





0.2l9-in-diam 

mounting holes 

(4) 



Figure 17.-Prototype sensitive GFR enclosure. 



R 6 , R 7 , and capacitor Q. It had a 3-dB cutoff frequency 
of 2 kHz and a gain of 2. Full-wave rectification was 
accomplished by the combination of the two operational 
amplifiers QA 3 and QA 4 . Diode D s , resistor R 14 , and 
capacitor C 5 provided a smoothed dc output signal 
comparable to the peak value of the input sinusoidal 
voltage at QA 3 . The smoothed signal was then applied to 
the noninverting input of the comparator QAj. This stage 
was designed such that its output state changes abruptly 
from zero to a high voltage when the input reaches an 
adjustable pickup reference voltage. 



QAi 

Amplifier 



QA 2 
Filter 



QA 3 , QA 4 
Full- wave 
rectifier 



R-C 
Integrator 



QA 5 
Level 
detector 



R-C, Q, 
Definite- 
time delay 



Open-CT 
trip signal 



Qz 
Trigger 



-ojp- 

Reset 



Target 



Trip 
relay 



:N.C 
:N.O. 



Figure 18.- Block diagram for prototype ac analog relay. 



During normal operation, the current from the 24-V 
supply flows through the coil of the normally closed elec- 
tromechanical relay K ls resistor R^, and the green light- 
emitting diode (LED) D 14 , because the thyristor, Q 2 , is off. 
When a ground fault occurs above the pickup setting, the 
voltage signal from the comparator starts to charge capaci- 
tor C 6 through the adjustable resistance R^ and fixed re- 
sistor R^. This continues until the unijunction transistor 
(UJT) turns on and discharges C 6 . The GFR operating 
time is determined by the time it takes to turn on the UJT. 
To activate the GFR, a trip signal greater than the trip 
level setting must still be present after the desired delay 
has occurred. The output of the UJT initializes the gate 
of the thyristor, Q 2 . Once the thyristor is fired, it latches 
on. This deenergizes the electromechanical relay and 
turns on the red LED D 13 . 

Several fail-safe characteristics were incorporated into 
the relay design. For example, a loss or significant drop of 
control power deenergizes the electromechanical relay coil. 
In addition, if the secondary winding of the CT disconnects 
from the analog circuitry, the relay will initiate a tripping 
signal. 

An internal power supply provides regulated ±15-V 
power for the relay's active integrated circuits, as well as 
an unregulated supply of approximately + 24 V for the re- 
lay K, and its associated network. The 120-V primary of 
the power supply transformer is fuse protected and in- 
cludes transient suppression. 

A test circuit, consisting of a test winding on the CT T 2 , 
momentary switch S 2 , and a current-limiting resistor R 33 
is provided on the relay. When the switch is closed, a test 
current greater than the relay pickup setting is primary 
injected into the CT, and the relay activates. 

Digital Version 

A block diagram of the digital prototype is shown in 
figure 20 and a schematic in figure 21. As with the analog 
version, the CT T 2 is connected across a 1,000-Q bur- 
den R 2 . The ac output voltage of the CT is amplified 
by the inverting operational amplifier QA 1B . The output 



10 




I k,Q 



+ 15 V 



R20 
10 Mil 



R|5 



R22 

22kft IOkil : 
-W\l — — + I5V' 



K-@TP 3 Time ± 
R23 delay ■ 
500 kU adjustment 




-@TP, 




/ \ 

D| 3 D| 4 
(Red) (Green) 



X C10 

0.1 uF ■= 
35 V 
"~ ""-15 V 



MC79LI5CP 



Figure 19.-Schematic diagram for prototype ac analog relay. 



OAia 
0pen-CT 
detector 



QA IB 

Zero- sequence 

amplifier 



QA 2A 
Filter 



QA 2B 
Level detector 



Time delay 
detector 



Qi 

Trigger 



4 i 



-0J0- 

Reset 



N.O. 



Target Trip 
relay 

Figure 20.-Block diagram for prototype digital relay. 

of QA 1B is then fed into a first-order, low-pass filter with 
a 3-dB cutoff of 1.6 kHz. The purpose of this filter is to 
attenuate high-frequency signals such that false tripping 
due to harmonic currents or high-frequency, long-duration 
transients may be avoided. This attenuation should not be 
too steep, because high-frequency currents can cause elec- 
trical fatalities. 



The ac output of the filter network is fed to the com- 
parator QA 2B through a dc-blocking network consisting of 
Q, R 12 , and C 4 . Consequently, an ac-filtered trip signal 
will be delivered to the negative input terminal of this dif- 
ferential amplifier. The voltage divider consisting of R 13 
and R 14 provides a positive reference voltage to the posi- 
tive input terminal of the differential amplifier. When the 
sinusoidal trip signal at the negative amplifier input is less 
than the trip level setting at the positive input terminal, 
the output of the comparator is at its positive saturation 
value of about + 13 V dc. At any instant when the trip 
signal is greater than the trip level setting, the comparator 
output goes to its negative saturation value of -13 V dc. 
During this time, diode CR 5 clamps the inputs to the 
CD4528 retriggerable one-shot and the 555 timer to ap- 
proximately V. 

The retriggerable one-shot remains in its stable state 
of unity logic output as long as a l-to-0 transition does not 
occur at its input. However, when such a transition does 
occur (corresponding to a trip signal), the retriggerable 
one-shot goes to its unstable state of zero logic output for 
as long as the trip signal is present. Should the input 
switch back to a high state before the GFR activates, the 
one-shot will return to its stable state and GFR operation 
will be blocked. 



11 




Test © 

winding 



Figure 21 .-Schematic diagram for prototype digital relay. 



The GFR time delay circuitry includes the 555 timer, 
the IQu NOR gate, and the CD4528 retriggerable one- 
shot. As long as no l-to-0 transitions occur at the timer 
input, its output remains in a stable zero state. When a 
l-to-0 transition does occur (corresponding to a trip ini- 
tiation), the 555 timer output becomes unstable (1) for 
100 ms, the GFR intentional delay time. The IC^ NOR 
gate isolates the timer output from the CD4528 one-shot 
input. 

The relay firing circuitry includes the IC 5A NOR gate, 
the 2N1595 thyristor, Q u the electromechanical relay K 1} 
and associated components. The relay tripping signal, a 
high NOR output, is developed only if the outputs of both 
CD4528 one-shots are zero. This occurs only if the relay 
delay time has expired and the trip signal exceeds the trip 
level setting. The remainder of the firing circuit is similar 
to the analog version, as are the power supply and test 
circuit. The digital version also has open-CT and loss-of- 
control-power safety features. 

Four analog and four digital prototypes were con- 
structed under phase 1 of Bureau contract J0134025 (19). 
Their ability to detect low-level faults was confirmed in the 



laboratory. What remained to be demonstrated was satis- 
factory operation in an underground mining environment. 

FIELD AND LABORATORY EVALUATIONS 

To ensure an unbiased evaluation, the Bureau assumed 
the responsibility of testing the units at a cooperative 
commercial mine. This demonstration would have a two- 
fold purpose: 

1. To expose the prototype GFR's to the transients 
and anomalies associated with a mine power system to 
gauge circuit durability. 

2. To measure the number of tripping events and de- 
termine if false or nuisance tripping is of concern when 
GFR's are actually providing ground-fault protection. 

Bruceton Mine 

Initially, the economically depressed coal industry 
was unresponsive to published solicitations for coopera- 
tors. Consequently, the devices were first installed at the 



12 



Bureau's Bruceton Mine near Pittsburgh, PA, to establish 
some performance history. 

Bruceton Mine Power System 

Electrical power at a potential of 7,200 V is delivered to 
the mine substation on the surface, as shown in fig- 
ure 22. The three-phase, 480-V utilization voltage for the 
mine is derived via a delta-wye configuration. Ground- 
fault currents are limited to 15 A by an 18.5-0 resistor 
connected between the secondary neutral point and the 
earth grounding bed. From the substation, power feeds 
into the surface control building (building 07) where it 
divides to supply both the Experimental Mine and the 
Safety Research Coal Mine (SRCM). Overcurrent, short- 
circuit, undervoltage, and grounded-phase protection are 
provided at the beginning of these branch circuits, located 
within a wall panel in building 07. The sole ground-fault 
protective device (GFR 2) for the Experimental Mine is in 
this panel. From building 07, power for the SRCM is 
transmitted underground through a borehole. A 480-V, 
three-phase service disconnect is situated underground at 
the bottom of this borehole. From this disconnect, power 
is fed into A-Butt, where three-phase auxiliary and single- 
phase lighting power is tapped (fig. 23). The auxiliary 
circuits, infrequently used to maneuver equipment in and 
out of the mine, are equipped with grounded-phase pro- 
tection (GFR's 7 and 8). In 12-Room, the 480-V power 
connects to a load center (fig. 24) for the portable mining 
equipment. Grounded-phase protection is incorporated on 
each of the four outgoing circuits. 

In summary, as shown in figure 22, ground-fault 
protection is provided at eight locations within the 
Bruceton Mine power system. Toroidal transformers 



7,200/480 V 



r~ 



_Buildlna _07 



Surface 
substation 



©) (D) 



Safety Research Coal Mine 

-Load center 
3-phase, 480-V 



12-Room 




A-Butt 



!®;)@WW) ! 

[-1-1-1 — I_J 



-L 



I 

Roof 
Loader bolter 



Cutter 



Auxiliary 
3-phase power 



Experimental 
Mine 




Auxiliary 
fan 



encircling the power conductors are connected to small 
socket-type relays with sensitivities of 6 A. 

Ground-Fault Relay Installation 

To preclude extensive tripping of the power, it was in- 
tended that the prototype sensitive GFR's operate event 
counters in lieu of activating the circuit breakers each time 
leakage current exceeded 50 mA. The existing ground- 
fault protection and grounding resistor remained in service. 
Simply connecting the unit's control power to the mine 
power system and exposing it to anomalies and transients 
constitutes one test of mine worthiness. 

Eight prototype GFR's were available for the demon- 
stration. Since their durability was unknown at the outset, 
it was decided to utilize only six, keeping two as spares. 
Consequently, no sensitive GFR's were installed on the 
rarely utilized auxiliary circuits in A-Butt. Two analog and 
two digital versions were installed underground in the load 
center; two digital units were placed in building 07. 




Figure 23.-Power feedthrough in A-Butt, Bruceton Mine. 




Figure 22. -Bruceton Mine power system. Circled numbers 
indicate GFR's. 



Figure 24.-Load center in 12-Room, Bruceton Mine. 



13 



All prototype GFR's installed underground were ad- 
justed for a definite time delay of 100 ms, the recom- 
mended setting for shock protection. To incorporate 
a degree of coordination, the upstream devices in build- 
ing 07 were fixed with a delay of 250 ms. 

As shown in figure 25, the CT's of the sensitive GFR's 
were installed around the three 480-V phase conductors. 
The single-phase control power for the electronic relays 
was obtained using 480/120-V control transformers. The 
electromechanical counters were rated at 120 V and 6 W. 

Upon installation, the test circuit of each GFR was ex- 
ercised to verify operation. At each location (building 07 
or the 12-Room load center) all relays tripped when the 
test button of any one was activated. The following were 
investigated as possible sources of this false tripping: 

• Physical orientation of the relays (because of space 
limitations the units were installed on their sides in the 
load center), 

• Induced currents in parallel CT secondary leads, 

• Low control voltage, 

• Voltage transient from the counter activation. 

The GFR's were tested in the laboratory with the relays 
upright, on their sides, upside down, etc.; the sensitivity 
and time delay were unaffected. Next, two GFR's were 
energized with their CT secondary leads at various dis- 
tances and orientations with respect to each other. No 
differences in performance were detected. Voltage mea- 
surements of the control transformer underground showed 
it remained constant at 120 V when the test buttons were 
pushed. Further tests in the laboratory revealed that the 
relays would not trip until the control voltage dropped 
to 65 V. Finally, the counters were disconnected, and 
the relays operated properly without interfering with 
each other. These counters consist of an electromagnetic 
coil that, upon loss of signal, counts by disengaging a 
ratchet wheel. The switching of this inductive load created 
a voltage transient that activated the other units. 




Several methods of transient suppression were tried in 
the laboratory without success. Metal-oxide varistors 
(MOV's) were connected across the input of the relay 
electronics; capacitors were placed across, and ferrite-core 
inductors in series with, the relay power supply. Only the 
placement of 0.33-^F capacitors across the counter coil 
solved the problem. 

It was felt that GFR transient immunity was not com- 
promised by the tripping associated with the electro- 
magnetic counters. The voltage transient created by the 
inductive coil switching was severe, but extremely localized 
in the power system. The switching of large motor loads 
should not give rise to similar problems because of the 
damping effect of the intervening cable impedance. 

Ground Fault Relay Performance 

The relays were then connected to the mine power sys- 
tem, with mine personnel periodically inspecting the units 
and resetting those that tripped. Table 1 lists the GFR 
performance at the end of 30 days. 

Table 1. -Counter readings of GFR performance 



Relay location 1 


1 


2 


3 


4 


5 


6 


After 30 days 


3 
1 


9 
3 


5 
3 


1 



2 
1 


n 


After additional 2 weeks . . . 





'See figure 22. 















480-V, 
3-phase 

Figure 25.-Sensltlve GFR Installation, Bruceton Mine. 



At this time all units were removed from service and 
examined in the laboratory. Both digital units installed in 
building 07 were malfunctioning. GFR 1, protecting the 
SRCM, could not be reset and remained in a trip mode. 
The GFR for the Experimental Mine would not trip when 
tested. After troubleshooting, the following components 
(fig. 21) were found defective and repaired: 

GFR 1: Digital IC 5 , CD4001. 

GFR 2: Thyristor Q^ 2N1595; reset switch S,; in addi- 
tion, the main power transformer Tj was resoldered. 

Before the evaluation was continued underground, the 
prototypes were thermally stressed at 120° F while ener- 
gized in an environmental chamber for 1 week. No addi- 
tional malfunctions were uncovered. The units were then 
reinstalled at their original locations on the mine power 
system and monitored for 2 weeks by mine personnel. The 
largest motor fed from the load center, the 60-hp, 480-V 
pump motor for the cutting machine, was started repeat- 
edly to determine if the resultant current inrush would 
affect GFR 3. No false trips were observed. The ab- 
sence of tripping when the pump motor was started is 
encouraging from the standpoint of immunity of transient 
common-mode currents. At the end of this time, digital 
GFR 5 was found inoperative because of defective IC 5 , 
CD4001. In addition, it was observed that when the relay 
electronics cards were inserted into the relay enclosures, 
they did not always contact the rear terminals without 
being jiggled. 



14 



The prototype ac GFR's were installed on the Bruceton 
Mine power system for a total of 6 weeks. During this 
period they were exposed to anomalies and transients as- 
sociated with the system. The count history revealed a 
high number of trips for the GFR protecting the 
Experimental Mine (GFR 2, table 1). It may be specu- 
lated that the relays in building 07 were more accessible 
than those underground and were reset with a higher 
frequency by mine personnel. Also, it may be speculated 
that the malfunctions of the digital relays in building 07 
were the result of a design flaw or their greater exposure 
to upstream power system transients, especially lightning. 
Additional studies were necessary to pinpoint the causes of 
these malfunctions. 

Ground Fault Relay Modifications 

The 6-week evaluation at the Bruceton Mine pointed 
out several deficiencies in the test setup and the GFR 
design, which were addressed in followup laboratory work. 
First, the use of 120-V electromechanical counters to 
monitor relay activation was judged to be of limited utility, 
since mine personnel were relied upon to check the units 
visually. An unattended GFR may count one trip when in 
reality it could have tripped many times with prompt reset. 
Consequently, electronic totalizers were procured as 
replacements for the electromechanical counters. Char- 
acterized by high input impedance, the totalizers featured 
an internal battery power supply and were sensitive to 6-V 
pulses. These monitoring devices were connected to the 
GFR's internal circuitry via amplifier circuits shown in 
figure 26. The totalizers monitor the presence of tripping 
signals, internal to the GFR, that are present any time 
leakage currents through the CT exceed 50 mA for the 
prescribed time delay. Since the internal tripping signals 
are unaffected by relay status, trip events can be monitored 
without reliance on mine personnel. 

As shown in figure 21, outputs 3 and 11 of NOR 
gate ICjg were grounded. However, through inversion 
these outputs became high and should be floated. Cor- 
rection of this error eliminated the overheating of 
ICjb (CD4001) in the digital GFR models. 

In addition to these basic changes, further modifications 
were recommended. Troubleshooting in the laboratory 
was hampered by the lack of readily identifiable test points 
on the printed circuit board. Key circuit locations could be 
made more accessible by the installation of color-coded 
test points on the side of the board. Also, to permit 
measurements while the GFR is in service, extender 
boards could be fabricated. 

Extracting the printed circuit board from the metallic 
enclosure opens the CT secondary. During service, high 




2N2222 

To 

k ,Q<L counter 



KEY 
[3] [JJ Test points on ac GFR 

[JJ Emitter of UJT Q, 

U Output of QA 5 (IC 2 ) (PIN 7) 

Figure 26.-Totalizer amplifier circuits for digital (A) and analog 
(S) GFR's. 

voltages could be present across these terminals, endan- 
gering personnel in close proximity. Consequently, a 
means should be devised to ensure that the CT secondary 
is shorted if the circuit board is removed. This could take 
the form of a shorting connector or a high-ohmic-value 
resistor in combination with back-to-back Zener diodes to 
limit high voltages. 

To ease CT installation, metal mounting brackets and 
screw terminals should be added to the CT housing. Also, 
a connector should be installed on the relay enclosure to 
facilitate connection of the CT and the 120-V control 
power. 

Many modifications can be implemented to improve 
ruggedness and reliability. The CT assembly can be pot- 
ted. To improve connector contact, the circuit board can 
be double sided with plated through-holes. In lieu of 
commercial-grade components, parts built to military 
specifications can be employed. 

Many of these changes were incorporated subsequently 
in GFR's built for the Bureau by a small electronics firm. 
These prototypes were then used in field evaluations at 
commercial sites. 



15 



Commercial Tests 

Through announcements in trade magazines and the 
Bureau's "Technology News," the mining community was 
made aware of the potential safety benefits of sensitive 
ground-fault protection. Several companies inquired about 
the availability of prototypes for field evaluations. 

Three GFR's were forwarded to a western haulage- 
equipment manufacturer for testing on two types of new 
electrical scooptrams used in hard-rock mines. One 
vehicle, intended for use in small-diameter haulageways, 
had a tramming capacity of 1,500 lb with a 0.5-yd 3 bucket. 
Electrically powered at 380 V, three-phase, 50-Hz, via a 
250-ft, No. 6 AWG trailing cable, it featured a 30-hp 
motor onboard. Existing ground-fault protection was set 
at 4 A with no fault-limiting resistance. With the sensitive 
GFR trip set at 60 mA, the vehicle was operated through 
load-haul-dump cycles under start, run, and stall conditions 
for over 8 h without any GFR trips. 

Next, a prototype GFR was evaluated with a large 
scooptram of 27,000-lb tramming capacity and 8-yd 3 bucket 
capacity. Electrically powered at 600 V, three-phase, 
60-Hz, it featured a 200-hp motor onboard. Nuisance-free 
GFR operation was experienced with a trip setting of 
100 mA. When adjusted to lower settings, the GFR would 
activate upon motor startup. 

These tests, the first commercial evaluations of the 
prototype GFR's, proved their viability for 50-Hz power. 



It is believed that further reductions in the trip setting 
would have been possible if a grounding resistor of 
suitable ohmic value had been utilized at the source 
transformer. Nevertheless, the manufacturer was quite 
pleased with the results and noted that a high degree of 
safety was obtained with protection set at 100 mA. This 
manufacturer is now considering equipping all new vehicles 
with sensitive GFR's. 

Through assistance from the Mine Safety and Health 
Administration, arrangements were made with a mine 
operator to evaluate the sensitive GFR in an underground 
coal mine in southern Ohio. The prototype was installed 
within a 1,040-V ac load center and protected a 750-ft, 
1/0 AWG, SHD-GC cable powering a continuous miner. 
When the power was energized, the relay immediately acti- 
vated, indicating a ground fault was present. Trouble- 
shooting by the mine electrician revealed that the high- 
frequency signal of the local ground-check monitor, 
superimposed on the power conductors, was induced into 
the relay through the GFR toroidal CT. Repositioning the 
monitor's tone filter eliminated this problem. However, 
the sensitive GFR continued to activate whenever the 
circuit breaker was closed. This was attributed to the 
significant charging current of the shielded cable and was 
eliminated when the GFR trip setting was increased to 
75 mA. 



DIRECT CURRENT UTILIZATION 



The second phase of the Bureau's sensitive ground-fault 
research program focused on protection of dc power 
systems feeding offtrack vehicles. For the past 25 years 
the use of dc for powering underground m inin g equipment 
has continually declined, with a concomitant rise in the 
consumption of ac. This trend is logical as ac can be 
transmitted and distributed more efficiently and ac motors 
require much less maintenance. At first glance, it may 
seem that dc systems in the mining industry are destined 
for obsolescence. However, for certain mining applications 
such as traction where high starting torques are needed, 
the series-wound dc motor is more suitable. Thus, dc will 
continue to play a role in powering m inin g machinery, 
especially in the form of onboard rectification. Workers 
will continue to operate and repair dc machines, and the 
dc shock hazards associated with these jobs will continue 
to exist. 

Previous Bureau-sponsored research (9) showed that of 
all possible ground-fault protection schemes, a differential- 
current arrangement using a saturable transformer 
appeared to provide the greatest measure of safety because 
of its fail-safe operation and selectivity in protecting 



individual circuits. As shown in figure 27, a toroidal trans- 
former serves as the ground-fault current sensor and 
encircles both the positive and negative outgoing con- 
ductors of the dc circuit. The primary winding of the 
sensor is excited by an ac signal, while the secondary is 
connected to the GFR. 



Saturable 
transformer 



To 



rectifier _^_ 



To grounding 
resistor 




To ac 
primary excitation 



■*■ Positive 
— Negative 
-*■ Ground 



To 
machine 



Figure 27.-Differential current relaying using saturable 
transformer. 



16 



Under unfaulted conditions, ac is induced in the 
secondary winding, causing the relay to operate in the 
normal mode. The circuit current through the positive 
conductor equals that through the negative conductor, and 
the magnetic fields about both conductors tend to cancel 
each other. When a ground fault occurs, the currents in 
the positive and negative conductors become unequal. The 
resultant magnetic field alters the transformer action of the 
sensor and reduces the voltage across the transformer- 
secondary winding, causing the relay to initiate a tripping 
action. These two components, the saturable transformer 
and the solid-state relay, comprise the sensitive dc ground- 
fault protection. 

PROTOTYPE DESIGN AND CONSTRUCTION 



Primary excitation frequency was plotted against the 
drop in output voltage during a ground fault. From fig- 
ure 30, it can be seen that a relatively low frequency is 
desirable. A nominal value of 110 Hz was chosen to avoid 
interference with 60- and 180-Hz induced voltages. 

The completed design is shown in figure 31. The sen- 
sor's magnetic properties are mechanically protected by a 
metal and phenolic housing. 

Electronic Relay 

Analysis of the electronic relay shows that it consists of 
a series of building blocks (fig. 32). The 110-Hz oscillator 
powers the primary of the saturable transformer sensor. 



Current Sensor 

The sensor was designed (20) using theoretical predic- 
tions of performance along with experimental observations. 
The parameters to be quantified are shown in figure 28. 
They include R s , the series resistance; N p and N s , the 
primary and secondary turns; the core material and 
dimensions. A 0.5-in, tape-wound core was constructed of 
a high-permeability, low-loss nickel-iron alloy. To ac- 
commodate two power conductors, the core was sized at 
2.5 in; its overall outside diameter was 3.5 in. After 
winding, the core was annealed to remove strain and 
impurities from the tape. 

Dc was then applied through the window while the ac 
secondary voltage was measured, with the objective to 
maximize the drop in ac output for dc fault currents in the 
milliampere range. By varying the number of primary and 
secondary turns, the series resistance, and the exciting 
voltage, plots of the ac output versus the dc fault current 
were derived, as shown in figure 29. Satisfactory per- 
formance was obtained with a series resistance of 470 ft 
and 25 turns on the primary and secondary. 




Relay 
input 

impedance 




200 300 

FAULT CURRENT, mA 



500 



Figure 29.-Saturable transformer output voltage versus dc 
fault current 




dc load 



0.001 001 0.1 1.0 10.0 100.0 

FREQUENCY, kHz 



Figure 28. -Saturable transformer current sensor. 



Figure 30.-Drop in sensor output versus frequency. 



17 





Si 



F 



Figure 31 .-Saturable transformer prototype. 




|ov 



+ I2V 



Adjustable 
time delay 



IIO-Hz 
oscillator 



Trip level 
filter output 



Comparator 
output 



Trip signal 
sensor output 



Delay timer 
output 



Oscillator 



1 Current 
"1 sensor 

I 



Band- pass 
filter 



Comparator 



















Trip signal 
sensor 


— *■ 


Relay delay 
timer 


— ». 


Relay trip 
network 




Mechanical 
relay 


i 


i 








\ 


' 




t 






Reset check 
network 






Power 
supply 















Figure 32.-Dc relay block diagram. 



Figure 33 displays timing diagrams from the circuitry 
shown schematically in figure 34. During the imposition 
of a ground fault, the magnitude of the primary input 
signal remains constant. The sensor output, essentially 
identical to the input, feeds a second-order band-pass filter 
designed with a 3-dB bandwidth of 10 Hz and a midband 
voltage gain of 5. When a ground fault greater than or 
equal to 150 mA dc flows through the sensor window, the 







Trip signal 
generator 



Figure 33.-Dc relay timing diagram. 



sensor core saturates and the fdter output drops to 55 pet 
of its original value. The peak ac output of the band-pass 
fdter is compared with a positive reference voltage at 
the input of a comparator. Adjustment of this reference 
voltage sets the current pickup or trip level, recommended 
at 150 mA dc. 

During a fault the comparator no longer sends periodic 
pulses to a retriggerable one-shot acting as a trip signal 
sensor. Accordingly, the output of this one-shot switches 
from 1 to 0, its stable state. This transition triggers an 
adjustable timer, which in turn activates the relay trip 
network via the gating of a silicon-controlled rectifier 
(SCR), if the fault duration is 100 ms or longer. The firing 
of this thyristor diverts current from the coil of an 
electromechanical relay. This deactivated relay in turn 
operates the trip mechanism of the circuit breaker. As 
with the ac sensitive GFR's, the dc prototype features a 
reset switch and a test circuit. In addition, the GFR acts 
to remove power upon loss of signal from the sensor or 
upon loss of the 120-V ac control power. 



18 



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cr.<t 
o z 



5 ,-KHi' 



cj 



*— otr> 



rose 





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a 



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19 



The components of the prototype are mounted on an 
interchangeable board within a metallic housing as shown 
in figure 35. This compact design measures 8-3/4 in high 
by 5-1/2 in deep by 2 in wide. 

LABORATORY EVALUATION 

The sensitive dc GFR prototypes were evaluated for 
immunity against voltage and current transients in a 



manner similar to that used for the ac prototypes. In all 
tests, the relays did not falsely activate nor did they 
become damaged by the simulated power system surges. 
The dc GFR's are available for installation in an 
underground mine having dc face equipment fed from a 
three-phase full-wave rectifier. 



ALTERNATING CURRENT DISTRIBUTION 



The final phase of the Bureau's ground-fault research 
program focused on the ac distribution portion of coal 
mine power systems. These high-voltage circuits feature 
a series of switchhouses, each with inherent ground-fault 
protection, distributed along the way from the surface to 
the section load centers. The operation of the electro- 
mechanical GFR's, installed in these switchhouses, is 
coordinated by large, intentional time delays to ensure that 






faults are cleared without interrupting power to sound, 
upstream circuit portions. These additive delays may in 
practice average 2 s per relay. As a result, upstream relays 
respond slowly to hazardous ground currents and workers 
are exposed to fire and burn hazards. In response to these 
problems, the Bureau conducted research to design and 
construct a coordination-free relaying system that reacts to 
a ground fault 50 to 100 times faster than present pro- 
tective systems. 

To understand the concept of coordination-free re- 
laying, a series of switchhouses must be visualized installed 
along a high-voltage distribution line (fig. 36). If a relay 
in an upstream switchhouse detects a ground fault and a 
downstream relay does not, the fault must be in the zone 
joining them. Consequently the upstream relay will acti- 
vate the local circuit breaker. If both upstream and down- 
stream relays detect faults, the fault must be downstream 
from the zone and neither relay will activate. 

BACKGROUND RESEARCH 

Initial research centered on determining the highest 
practical ohmic values for high-voltage grounding resistors. 
These components, connected between the transformer- 
secondary neutral point and the earth grounding medium, 
restrict ground-fault current magnitude. To provide pro- 
tection against electrocutions, this limitation should be in 
the milliampere range. 

Underground high-voltage cable is required to have 
metallic shielding around each power conductor. Con- 
sequently, this cable exhibits significant distributed 



Figure 35. -Dc relay prototype. 




SH I SH 2 SH 3 

Figure 36.-Typical high-voltage distribution circuit 



20 



capacitance not only among power conductors, but also 
between power and ground conductors. In addition, the 
use of discrete capacitors for surge and power-factor 
correction adds to the circuit capacitance. Thus, dis- 
tribution circuits feature charging currents of up to 10 A. 
Through a computer-assisted transient evaluation of mine 
distribution circuits, it was concluded that instability results 
if ground faults are limited below capacitive charging 
currents. Thus, it was recommended that present ground- 
fault-limiting levels of 25 A continue to be utilized, 
precluding complete personnel shock protection. 

With coordination-free relaying, detected-fault status is 
continuously transmitted from the downstream to the up- 
stream relay. Utilizing dependable blocking logic, the 
signal from the downstream relay acts to prevent tripping. 
The most practical communication link available appears 
to be the ground-check conductors already present in 
almost all high-voltage cables used with pilot ground-check 
monitors (GCM's). 

Ground-check monitoring systems for high-voltage dis- 
tribution usually monitor the integrity of the grounding 
conductor connected between switchhouses (fig. 37). 
Separate, independent GCM units are installed in each 
switchhouse. GCM's commonly circulate low-voltage 
60-Hz signals through the pilot-ground circuit. Con- 
sequently, a transmission frequency of 1,000 Hz was 
chosen for the coordination-free ground-fault relay 
(CF-GFR) blocking signal. Active electronic filters were 
used to couple and decouple the superposed signals. 

Two intentional delays were proposed for the ex- 
perimental relays: 50 ms for primary and relay-racing 
protection and 150 ms for backup protection. Primary 
protection must include time for fault signal detection and 
a safety factor. Relay racing occurs if a blocking signal is 
not received in time to prevent false tripping upstream. 
The backup delay is desirable should the downstream 
breaker fail to open during a ground fault. 

It was recommended that backup be graded in 150-ms 
increments, with the most downstream breaker in a series 



path having a 50-ms delay as primary protection and no 
backup relay. The next outby breaker would have a 50-ms 
delay for primary protection and a total delay of 200 ms in 
backup. The next relay would exhibit delays of 50 and 
350 ms, respectively. 

PROTOTYPE DESIGN AND CONSTRUCTION 

Once design criteria were established, a CF-GFR was 
designed and three prototypes were constructed. Block 
diagrams for the overall system and each relay are shown 
in figures 38 and 39, respectively (27). The CF-GFR 
design has been fully documented through schematics, wir- 
ing diagrams, and component listings (22). 

The blocking signal, tuned to 1,000 Hz, is coupled to 
the pilot and ground wires through a switch controlled by 
a logic device. When a ground fault is sensed locally, the 
coupling is accomplished. The purpose of the attenuator 
is to avoid a very low impedance path between the pilot 
and ground wires. Such a path would severely reduce the 
blocking signal amplitude and prevent its propagation to 
the upstream switchhouse and relay. 

The upstream detector circuit checks for proper ampli- 
tude and frequency in the incoming downstream blocking 
signal. The 60-Hz GCM signal is attenuated by the band- 
pass filter. When the blocking signal is detected, a block- 
ing logic signal is sent to the trip element actuator to in- 
hibit primary protection tripping. 

Local zero-sequence ground faults are sensed by the 
toroidal CT. When this ground current is greater than the 
pickup level of 5 A, a trip signal is sent to the trip element 
actuator after the appropriate delay interval. However, 
primary protection is inhibited if a blocking signal has 
been detected from the downstream relay. Backup delay 
time is then sensed. The trip element is then actuated at 
the end of the backup delay interval if the locally sensed 
ground fault is still greater than the pickup setting. 

Additionally, if the local fault-sensing CT becomes 
open, then, irrespective of any other signals, a trip is 



Pilot 

conductor 



2 



Grounding 
conductor 



GCM 



J? 



Pilot conductor 



Grounding conductor 



GCM 



J) 



— Pilot 
conductor 



t 



Grounding 
conductor 



SH 1 



SH 2 



Power flow O 

Figure 37,-Simplified diagram of ground-check monitoring system used in mine distribution systems. 



GFR signal 
generator 



Switch 



Pilot 
wire 



Ground wire 



I, , 



Primary delay 
circuit 



Blocking 
logic 



Trip element 
actuator 



Backup delay 
circuit 



a 



Circuit breaker 



Attenuator 



Machine 
frame 



60-Hz GCM 



21 



GFR 
detector 



Logic 
device 



Band-pass 
filter 



Local fault 
sense circuit 



X 



Phase conductor 



GCM relay 



Pilot 
wire 



Ground wire 



Figure 38.-System block diagram. 



Phase 

conductor 



Sense and 
amplify 



Open-coil 
detection 



Filter 



Backup delay 

generator 

circuit 



T 



Logic for 
trip signal 
generation 



Adjust 

pickup level 



Comparator 



Adjust delay 



Primary delay 

generation 

circuit 



Local trip 

signal 
generator 



Logic for 
trip signal 
generation 



Trip 
element 



Downstream 
inhibit signal 



Figure 39.-Block diagram of relay circuit 



22 



activated instantaneously. Also, if an increase in imped- 
ance of the pilot- and ground-wire circuits is sensed by the 
GCM, an instantaneous trip is activated. 

LABORATORY EVALUATION 

Laboratory tests with a 12-V commercial GCM and the 
CF-GFR prototypes verified that both safety devices will 



function without interfering with each other. The operat- 
ing range of the GCM's, whose voltage is compatible 
with the GFR filters, is limited to 4,000 ft by the 
substantial impedance of No. 8 AWG pilot conductors. 
Nevertheless, this should not preclude the direct appli- 
cation of CF GFR's on most underground distribution 
circuits. 



CONCLUSIONS 



Sensitive and coordination-free ground-fault protec- 
tion has been designed for use on resistance-grounded 
mine power systems. To facilitate commercial manu- 
facture, the designs are fully documented by detailed 
schematics, assembly drawings, and component listings. 
Prototype units have been tested in the laboratory and 
are available for installation in underground mines. 
Implementation of this practical protection would not 
require extensive alterations to mine power systems. 



Existing GFR's would simply be replaced with solid-state 
units. 

The sensitive GFR's, when installed on ac and dc mine 
utilization circuits, can prevent nearly all the poten- 
tial electrocutions on these low-voltage power systems. 
CF GFR's, installed on ac distribution circuits in coal 
mines, can reduce the incidence of fires associated with 
high-voltage power systems by significantly decreasing 
response time to faults. 



REFERENCES 



1. Oyier, A. Electrical Accidents in Mining (1980-85). Fatal and 
Nonfatal Accidents Underground and on the Surface at Underground 
Coal and Metal-Nonmetal Mines. BuMines IC 9259, 1990, in press. 

2. Dalziel, C. F., and W. R. Lee. Reevaluation of Lethal Electric 
Current. IEEE Trans, and Ind. Gen. Appl., v. IGA-4, No. 5, 1968, 
p. 467. 

3. Lee, R L. Electrical Safety in Industrial Plants. IEEE Trans. 
Ind. and Gen. Appl., v. IGA-7, No. 1, 1971, p. 11. 

4. Geddes, L. A., and L. E. Baker. Response to Passage of Electric 
Current Through the Body. J. Assoc. Adv. Med. J - f rum., v. 5, No. 1, 
1971, p. 233. 

5. Knickerbocker, G. C. Fibrillating Parameters of Direct 
and Alternating 20 Hz Currents Separately and in Combination-An 
Experimental Study. IEEE Trans. Commun., v. COM-21, No. 9, 1973, 
p. 1017. 

6. Bernstein, T. Safety Criteria for Intended or Expected Non- 
Lethal Electrical Shock. Proceedings of the First International Sympo- 
sium on Electrical Shock Safety Criteria, Toronto, Canada. Pergamon 
Press, Inc., 1985, p. 283. 

7. Morley, L. A. Mine Power Systems (contract J0155009, PA State 
Univ.). Volume II. BuMines OFR 178(2)-82, 1982, p. 79; NTIS PB 83- 
120386. 

8. Wagner, C. F. Symmetrical Components. McGraw-Hill, 1933, 
437 pp. 

9. Morley, L. A., F. C. Trutt, and D. J. Rufft. Electrical-Shock 
Prevention (contract J0113009, PA State Univ.). Volume II-Ground- 
Fault Interrupting Devices. BuMines OFR 177(2)-83, 1982, 110 pp.; 
NTIS PB 84-102953. 

10. U.S. Navy. U. S. Standard Requirements for Electrical Equip- 
ment. Mil. Stand. 454J, Sec. 1 and 36, Apr. 1984, pp. 1-1-1-12 and 
36-1-36-2. 

11. Cablec Corp. (Indianapolis, IN). Mining Cable Engineering 
Handbook. 1987, 168 pp. 

12. Stanek, E. K., W. Vilcheck, and A. Kunjara. Mine Electrical 
Power Systems. Transients Protection, Reliability Investigation, and 
Safety Testing of Mine Electrical Power Systems (grant G0144137, WV 



Univ.). Volume I-Transients in Mine Electrical Power Systems. 
BuMines OFR 6(1)-81, 1979, 169 pp.; NTIS PB 81-166761. 

13. Underwriters Laboratories (Melville, NY). Ground-Fault Circuit 
Interrupters. Stand. 943, 1980, 14 pp. 

14. U.S. Code of Federal Regulations. Title 30-Mineral Resources; 
Chapter I-Mine Safety and Health Administration, Department of 
Labor; Subchapter 0-Coal Mine Safety and Health; Part 75-Mandatory 
Safety Standards-Underground Coal Mines, Subpart G-Trailing Cables; 
Sec. 75.601. July 1, 1988. 

15. Yenchek, M. R, and M. N. Ackerman. Evaluation of Sensitive 
Ground-Fault Interrupters for Coal Mines. BuMines IC 9057, 1985, 
15 pp. 

16. Magnetics (Butler, PA). Design Manual Featuring Tape- Wound 
Cores. Publ. TWC-300R 1980, 28 pp. 

17. Steen, F. L. Supertoroids With 'Zero' External Fields Made With 
Regressive Windings. Electron. Des., v. 18, Sept. 1976, p. 45. 

18. Dolinar, K. D. Improved Ground-Fault Protection System for 
Low- and Medium-Voltage Trailing Cables. Conference Record of the 
1980 IEEE-Industry Applications Society Annual Meeting, Cincinnati, 
OH. IEEE, 1980, p. 594. 

19. Morley, L. A, F. C. Trutt, and T. Novak. Sensitive Ground-Fault 
Protection for Mines. Phase I-Alternating Current Utilization, 
(contract J0134025, PA State Univ.). BuMines OFR 26-85, 1984, 89 pp.; 
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20. Kohler, J. L., F. C. Trutt, and L. A. Morley. Sensitive Ground- 
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(contract J0134025, PA State Univ.). BuMines OFR 29-87, 1986, 81 pp.; 
NTIS PB 87-203410. 

21. Trutt, F. C, H. G. Rotithor, and J. L. Kohler. A Coordination- 
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State Univ.). BuMines OFR 2-89, 1988, 95 pp.; NTIS PB 89-143382. 



23 





APPENDIX 


[.-ABBREVIATIONS AND SYMBOLS 


ac 


alternating current 




PW 


pilot wire 


AWG 


American wire gauge 




Q 


thyristor 


C 


capacitor 




QA 


operational amplifier 


CF-GFR 


coordination-free ground-fault 


relay 


R 


resistor 


CR 


relay 




Rt 


body resistance 


CT 


current transformer 




R s 


grounding resistor 


D 


diode 




R, 


inner toroidal radius 


dc 


direct current 




Rl 


load resistance 


E 


RMS output voltage of transformer secondary 


R 


outer toroidal radius 


f 


frequency 




R s 


source resistance 


F 


fuse 




RMS 


root-mean-square 


GCM 


ground-check monitor 




S 


switch 


GFR 


ground-fault relay 




SCR 


silicon-controlled rectifier (thyristor) 


h 


toroid core width 




SH 


switchhouse 


I 


current 




t 


time 


*p 


RMS value of ground-fault current 


ti 


opera^g time of GFR 


IC 


integrated circuit 




t 2 


operating time of molded-case circuit breaker 


K 


electromagnetic relay 




T 


transformer 


L 


inductor 




TP 


test point 


LED 


light- emitting diode 




u 


core permeability 


MOV 


metal-oxide varistor 




U 


digital logic component 


N 


number of turns on secondary < 


winding 


UJT 


unijunction transistor 


N p 


primary turns 




v tt 


control voltage 


N s 


secondary turns 




v* 


dc voltage 


N.C. 


normally closed 




v ta 


line-to-neutral system voltage 


N.O. 


normally open 




z 


Zener diode 



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